Step 1: Enable The Console

First, go to the Options & Controls, and then click Game Options. In Game options, click Enable Console to Yes to use the console.

Step 2: Using The Console

The console is a hidden feature that beta testers and programmers use to change settings that most people can't change. Fortunately, it can also be used to access levels that you haven't reached yet, and to use cheats.
To get to Nazi Zombies, press ~ (Shift+ `), between tab and 1. Clicking should no longer have any effect, and a bar saying some numbers and COD_WaW> . To go to Nazi Zombies, Type:
/devmap Nazi_zombie_prototype
Caps doesn't matter. The Loading Screen from Nazi Zombies should show.
Now You Can Play Nazi Zombies!